Hazardous or volatile liquids such as gasoline are primarily transported in cargo tanks. Typically, a cargo tank is divided into four or five separate compartments, each of which has a pressure relief system secured to its upper wall consisting of an emergency pressure relief valve and a combined vacuum relief/normal pressure relief valve. Alternatively, the combined vacuum relief/normal pressure relief valve may be replaced by separate vacuum relief and normal pressure relief valves. The normal pressure relief feature of the combined vacuum relief/normal pressure relief valve provides minimal flow capacity and functions to alleviate minor increases in the cargo tank internal pressure by venting pressurized vapors to the atmosphere. Similarly, the vacuum relief feature of the combined vacuum relief/normal pressure relief valve provides minimal flow capacity but, unlike the normal pressure relief feature, functions to alleviate minor decreases in the cargo tank internal pressure by venting atmosphere into the tank compartment. The emergency pressure relief valve operates to protect the tank compartment from rupturing or exploding due to a significant increase in the cargo tank internal pressure which could not otherwise be alleviated by the normal pressure relief valve.
Significant increases in the cargo tank internal pressure requiring emergency venting most often occur during either (1) accidents resulting in fire or substantial tank deformation or (2) cargo tank loading when the vapor recovery system fails to vent cargo vapors from the tank compartments. Under such conditions, the emergency pressure relief valve is designed to automatically open when the internal tank pressure reaches a predetermined level, or "set pressure," to relieve the excess pressure within the tank compartment by venting pressurized vapors to the atmosphere. The value of this "set pressure" varies depending upon the maximum allowable working pressure for each of the different types of cargo tanks. For example, with regard to MC 306 or DOT 406 type tanks typically used for transporting flammable liquids such as gasoline, the emergency pressure relief valve "set pressure" is approximately 3-41/2 PSI.
Traffic accidents involving cargo tank vehicles transporting hazardous or volatile materials often result in the cargo tank overturning and impacting the ground (hereinafter referred to as "rollover accidents"). During such rollover accidents, a very sudden liquid surge occurs within the cargo tank compartment which causes a relatively short but extremely high dynamic pressure increase within the tank compartment (hereinafter referred to as "pressure spike"). Because such a pressure spike usually exceeds the predetermined pressure level at which conventional emergency pressure relief valves are designed to automatically open, hazardous or volatile liquids may spray or leak from the cargo tank compartment to the surrounding area thus presenting a dangerous situation. In an attempt to eliminate this dangerous situation, the United States Department of Transportation promulgated regulations which require that, as of Aug. 31, 1995, cargo tank compartments must be equipped with emergency pressure relief valves which leak no lading during such pressure spikes (commonly referred to as "zero leakage" regulations).
Presently, there is no known reliable technique to safely prevent emergency pressure relief valves from opening and leaking lading during pressure spikes resulting from rollover accidents. In response to the Department of Transportation zero leakage regulations, the industry has developed "zero leakage" emergency pressure relief valves which operate to prevent the leakage of lading during pressure spikes by momentarily closing for the duration of the pressure spike. However, because these emergency pressure relief valves are designed to remain closed during pressure spikes, the basic danger of tank rupture or explosion exists since no means is provided to relieve sudden increases in tank compartment pressures whether static or spike related. Further, because these emergency pressure relief valves are complicated in design and use dynamic pressure seals, such as O-rings, a danger exists that these valves might seize or clog, thus rendering the valve inoperable.
Moreover, because these emergency vents are further designed to open in response to excessive internal tank compartment pressures after momentarily closing for the duration of a pressure spike, a danger of lading leakage exists in the event of a rollover accident resulting in deformation of the tank compartment. It is commonly understood that the deformation of the tank walls results in a reduction of the tank volume. Moreover, since the tank compartment vapors are highly compressible relative to the liquid lading, any reduction in the tank volume results in an increase in the tank vapor pressure. For example, a tank deformation which reduces the tank compartment vapor space by one-half the original vapor space increases the tank compartment vapor static pressure by one atmosphere or 14.7 PSI. Thus, for a typical fully loaded cargo tank having a tank compartment vapor space of 3% of the total tank compartment volume, a 1.5% reduction in the tank compartment volume resulting from tank deformation increases the tank compartment vapor pressure by 14.7 PSI. Because emergency pressure relief valves on cargo tank vehicles transporting gasoline are designed to open when the internal tank pressure exceeds approximately 3-41/2 PSI, an increase in the tank compartment vapor static pressure by one atmosphere (14.7 PSI) due to tank deformation will cause the emergency pressure relief valve to open. Therefore, although these "zero leakage" emergency pressure relief valves momentarily close to prevent lading from leaking during pressure spikes, in the event of a rollover accident which results in tank deformation these "zero leakage" emergency pressure relief valves may subsequently open and permit lading to leak to the surrounding area until the internal tank pressure drops below about 3-41/2 PSI.
Thus, there currently exists a compelling and industry recognized need for an emergency pressure relief valve which provides a means for uninterrupted relief of excess internal cargo tank pressure and also a safe and reliable means for preventing leakage of lading, regardless of tank orientation, during pressure spikes resulting from rollover accidents or excessive internal tank compartment static pressure caused by tank deformation.